<details>를 열고 닫는 마커를 ➕와 ➖로 바꾸는 경우

index.html

<details>
  <summary>
    <h3>Summary Title</h3>
    <p>and summary description</p>
  </summary>

  <h3><a href="#">Item Title</a></h3>
  <p>and item description</p>
</details>

style.css

h3 a {
  display: block;
}

h3 + p {
  margin-top: 0;
}

details summary::-webkit-details-marker {
  display: none;
}

details summary::before {
  content: "\2795";
  float: right;
  text-align: right;
  margin-top: -.1rem;
}

details[open] summary:before {
  content: "\2796";
}